Elements

Elements from Compounds and Mixtures

Chemists, like curious children, learn about the world around them by taking things apart. Instead of dissecting music boxes and battery-operated rabbits, however, they attempt to dismantle matter, because their goal is to understand the substances from which things are made. The model of the structure of matter presented in the article on solids, liquids, and gases describes the behavior of the particles in a solid, a liquid, or a gas. But what about the nature of the particles themselves? Are all the particles in a solid, liquid, or gas identical? And what are the particles made of? We begin our search for the answers to these questions by analyzing a simple glass of water with table salt dissolved in it.

Figure 1. Water can be separated from salt water on a small scale using a laboratory distillation apparatus. (Source: Preparatory Chemistry)

We can separate this salt water into simpler components in a series of steps. First, heating can separate the salt and the water; the water will evaporate, leaving the salt behind. If we do the heating in what chemists call a distillation apparatus, the water vapor can be cooled back to its liquid form and collected in a separate container (Figure 1). Next, the water can be broken down into two even simpler substances–hydrogen gas and oxygen gas–by running an electric current through it. Also, we can melt the dry salt and then run an electric current through it, which causes it to break down into sodium metal and chlorine gas.

Thus the salt water can be converted into four simple substances: hydrogen, oxygen, sodium, and chlorine (Figure 2). Chemists are unable to convert these four substances into simpler ones. They are four of the building blocks of matter that we call elements, substances that cannot be chemically converted into simpler ones. (We will get a more precise definition of elements after we have explored their structure in more detail.) Water, which consists of the elements hydrogen and oxygen, and salt, which consists of the elements sodium and chlorine, are examples of chemical compounds. The mixture of salt and water is an example of a solution.

Millions of simple and complex substances are found in nature or produced in the chemical laboratory, but the total number of elements that combine to form these substances is much, much smaller. By the year 2003, 114 elements had been discovered, but 24 of these elements are not found naturally on the earth, and chemists do not generally work with them. Of these 24 elements, 2 or 3 might exist in stars, but the rest are not thought to exist outside the physicist’s laboratory. Some of the elements found in nature are unstable; that is, they exist for a limited time and then turn into other elements in a process called radioactive decay. Of the 83 stable elements found in nature, many are rare and will not be mentioned in this text.

Each of the elements is known by a name and a symbol. The names were assigned in several ways. Some of the elements, such as francium and californium, were named to honor the places where they were discovered. Some have been named to honor important scientists. An element discovered in 1982 has been named meitnerium to honor Lise Meitner (1878-1968), the Austrian-Swedish physicist and mathematician who discovered the element protactinium and made major contributions to the understanding of nuclear fission. Some names reflect the source from which scientists first isolated the element. The name hydrogen came from the combination of the Greek words for "water" (hydro) and "forming" (genes). Some elements, such as the purple element iodine, are named for their appearance. Iodos means violet in Greek.

The symbols for the elements were chosen in equally varied ways. Some are the first letter of the element’s name. For example, C represents carbon. Other symbols are formed from the first letter and a later letter in the name. When two letters are used, the first is capitalized and the second remains lowercase. Cl is used for chlorine and Co for cobalt. Some of the symbols come from earlier, Latin names for elements. For example, Na for sodium comes from the Latin natrium, and Au for gold comes from the Latin aurum, which means shining dawn. The most recently discovered elements have not been officially named yet. They are given temporary names and three-letter symbols. Table 1 contains contains names and symbols for common elements.

The Periodic Table of the Elements

Hanging on the wall of every chemistry laboratory, and emblazoned on many a chemist’s favorite mug or T-shirt, is one of chemistry’s most important basic tools, the periodic table of the elements (Figure 3). This table is like the map of the world on the wall of every geography classroom. If a geography instructor points to a country on the map, its location alone will tell you what the climate would be like and perhaps some of the characteristics of the culture. Likewise, you may not be familiar with the element potassium, but we shall see that the position of its symbol, K, on the periodic table, tells us that this element is very similar to sodium and that it will react with the element chlorine to form a substance that is very similar to table salt.

The elements are organized on the periodic table in a way that makes it easy to find important information about them. You will quickly come to appreciate how useful the table is when one knows just a few of the details of its arrangement.

The periodic table is arranged in such a way that elements in the same vertical column have similar characteristics. Therefore, it is often useful to refer to all the elements in a given column as a group or family. Each group has a number, and some have a group name. For example, the last column on the right is group 18, and the elements in this column are called noble gasesAny of the halogen elements: helium neon, argon, xenon, radon, krypton.

In the United States, there are two common conventions for numbering the columns (Figure 4). Groups 1 to 18: The vertical columns can be numbered from 1 to 18. This text will use this numbering convention most often. Groups A and B: Some of the groups are also commonly described with a number and the letter A or B. For example, sometimes the group headed by N will be called group 15 and sometimes group 5A. The group headed by Zn can be called 12 or 2B. Because this convention is useful and is common, you will see it used also. Some chemists use Roman numerals with the A- and B-group convention. In short, the group headed by N can be 15, 5A, or VA. The group headed by Zn can be 12, 2B, or IIB.

The groups in the first two and last two columns are the ones that have names as well as numbers. You should learn these names; they are used often in chemistry.

Most of the elements are classified as metals, which means they have the following characteristics.

Metals conduct heat well and in the solid form conduct electric currents.

Metals are malleable, which means they are capable of being extended or shaped by the blows of a hammer. (For example, gold, Au, can be hammered into very thin sheets without breaking.)

There is more variation in the characteristics of the nonmetal elements. Some of them are gases at room temperature and pressure, some are solids, and one is a liquid. They have different colors and different textures. The definitive quality shared by all nonmetals is that they do not have the characteristics mentioned above for metals. For example, sulfur is a dull yellow solid that does not conduct heat or electric currents well and is not malleable. It shatters into pieces when hit with a hammer.

A few of the elements have some but not all of the characteristics of metals. These elements are classified as metalloids or semimetals. Authorities disagree to some extent concerning which elements belong in this category, but the elements in yellow boxes in Figure 6 are commonly classified as metalloids.

The portion of the periodic table that contains the metallic elements is shown here in gray, and the portion that contains the nonmetallic elements is shown in light blue. The stair-step line that starts between B and Al on the periodic table and descends between Al and Si, Si and Ge, and so on separates the metallic elements from the nonmetallic elements. The metals are below and to the left of this line, and the nonmetals are above and to the right of it. Most of the elements that have two sides of their box forming part of the stair-step line are metalloids. Aluminum is usually considered a metal.

It is often useful to refer to whole blocks of elements on the periodic table. The elements in groups 1, 2, and 13 through 18 (the "A" groups) are sometimes called the representative elements. They are also called the main-group elements. The elements in groups 3 through 12 (the "B" groups) are often called the transition metals. The 28 elements at the bottom of the table are called inner transition metals.

The horizontal rows on the periodic table are called periods. There are seven periods in all. The first period contains only two elements, hydrogen, H, and helium, He. (The symbol for hydrogen is placed in different positions on different periodic tables. On some, it is placed in group 1, and on other tables, it is found at the top of group 17. Although there are reasons for placing it in these positions, there are also reasons why it does not belong in either position. Therefore, on our periodic table, it is separate from both groups.) The second period contains eight elements: lithium, Li, through neon, Ne. The fourth period consists of eighteen elements: potassium, K, through krypton, Kr.

Note that the sixth period begins with cesium, Cs, which is element number 55, and barium, Ba, which is number 56, and then there is a gap which is followed by lutetium, Lu, element 71. The gap represents the proper location of the first row of the inner transition metals—that is, lanthanum, La, which is element number 57, through ytterbium, Yb, which is element 70. These elements belong in the sixth period. Similarly, the second row of inner transition metals, the elements actinium, Ac, through nobelium, No, belong in the seventh period between radium, Ra, and lawrencium, Lr.

At room temperature (20 °C) and normal pressures, most of the elements are solid, two of them are liquid (Hg and Br), and eleven are gas (H, N, O, [F, Cl, and the noble gases).

The Structure of the Elements

What makes one element different from another? To understand the answer to this question, you need to know about their internal structure. If you were to cut a piece of pure gold in half, and then divide one of those halves in half again and divide one of those halves in half, and continue to do that over and over, eventually the portion remaining could not be further divided and still be gold. This portion is a gold atom. The element gold consists of gold atoms, the element carbon consists of carbon atoms, and so on. To understand what makes one element different from another, we need to look inside the atom.

The Atom

The atom is the smallest part of the element that retains the chemical characteristics of the element itself. For our purposes, we can think of the atom as a sphere with a diameter of about 10?10 meters. This is about a million times smaller than the diameter of the period at the end of this sentence. If the atoms in your body were an inch in diameter, you would have to worry about bumping your head on the moon.

Because atoms are so small, there are a tremendous number of them in even a small sample of an element. A ½-carat diamond contains about 5 × 1021 atoms of carbon. If these atoms, tiny as they are, were arranged in a straight line with each one touching its neighbors, the line would stretch from here to the sun.

If we could look inside the gold atom, we would find that it is composed of three types of particles: protons, neutrons, and electrons. (The physicists will tell you that the proton and neutron are themselves composed of simpler particles. Because it is not useful to the chemist to describe atoms in terms of these more fundamental particles, they will not be described here). Every gold atom in nature, for example, has 79 protons, 79 electrons, and 118 neutrons. Gold is different from phosphorus, because natural phosphorus atoms have 15 protons, 15 electrons, and 16 neutrons.

The particles within the atom are extremely tiny. A penny weighs about 2.5 grams, and a neutron, which is the most massive of the particles in the atom, weighs only 1.6750 × 10-24 grams. The protons have about the same mass as the neutrons, but the electrons have about 2000 times less mass. Because the masses of the particles are so small, a more convenient unit of measurement has been devised for them. An atomic mass unit (also called the unified mass unit) is 1/12 the mass of a carbon atom that has 6 protons, 6 neutrons, and 6 electrons. The modern abbreviation for atomic mass unit is ?, but amu is commonly used.

Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. Charge, a fundamental property of matter, is difficult to describe. Most definitions focus less on what it is than on what it does. For example, we know that objects of opposite charge attract each other, and objects of the same charge repel each other. An electron has a charge that is opposite but equal in magnitude to the charge of a proton. We arbitrarily assign the electron a charge of -1, so the charge of a proton is considered to be +1.

The Nucleus

Modern atomic theory tells us that even though the protons and neutrons represent most of the mass of the atom, they actually occupy a very small part of the volume of the atom. These particles cling together to form the incredibly small core of the atom called the nucleus. Compared to the typical atom’s diameter, which we described earlier as being about 10-10 meters, the diameter of a typical nucleus is about 10-15 meters. Thus, almost all the mass of the atom and all of its positive charge are found in a nucleus of about 1/100,000 the diameter of the atom itself. If an atom were the size of the earth, the diameter of the nucleus would be just a little longer than the length of a football field. If the nuclei of the atoms in your body were about an inch in diameter, you’d have to stand on the dark side of the earth to avoid burning your hair in the sun.

The Electron

If I seem unusually clear to you, you must have misunderstood what I said. – Alan Greenspan, Former Head of the Federal Reserve Board

Describing the modern view of the electron may not be as difficult as explaining the U.S. Federal Reserve Board’s monetary policy, but it is still a significant challenge. We do not think that electrons are spherical particles orbiting around the nucleus like planets around the sun. Scientists agree that electrons are outside the nucleus, but how to describe what they are doing out there or even what they are turns out to be a difficult task. One way to deal with this difficulty is to disregard the question of what electrons are and how they move and focus our attention only on the negative charge that they generate. Chemists do this with the help of a model in which each electron is visualized as generating a cloud of negative charge that surrounds the nucleus. In Figure 10, we use the element carbon as an example. Most of the carbon atoms in a diamond in a necklace have 6 protons, 6 neutrons, and 6 electrons. The protons and neutrons are in the nucleus, which is surrounded by a cloud of negative charge created by the 6 electrons.

Ions

Sometimes, when the elements form more complex substances, their atoms lose or gain electrons. Before this change, the atoms have an equal number of protons and electrons, and because protons and electrons have an equal but opposite charge, these atoms are initially uncharged overall. When an uncharged atom gains or loses one or more electrons, it forms a charged particle, called an ion. For example, when an atom loses one or more electrons, it will have more protons than electrons and more plus charge than minus charge. Thus, it will have an overall positive charge. An atom that becomes a positively charged ion is called a cation. For example, uncharged sodium atoms have 11 protons and 11 electrons. They commonly lose one of these electrons to form +1 cations. A sodium cation’s overall charge is +1 because its 11 protons have a charge of +11, and its remaining 10 electrons have a charge of -10. The sum of +11 and -10 is +1 (Figure 11). The symbol for a specific cation is written with the charge as a superscript on the right side of the element symbol. If the charge is +1, the convention is to write + (without the 1), so the symbol for the +1 sodium cation is Na+. Aluminum atoms commonly lose 3 of their electrons to form +3 cations. The cations are +3 because each aluminum cation has a charge of +13 from its 13 protons and a charge of -10 from its 10 remaining electrons. The sum is +3. The symbol for this cation is Al3+. (Notice that the 3 comes before the +.)

Some atoms can gain electrons. When an atom gains one or more electrons, it will have more electrons than protons and more minus charge than plus charge. An atom that becomes negatively charged due to an excess of electrons is called an anion, a negatively charged ion. For example, uncharged chlorine atoms have 17 protons and 17 electrons. They commonly gain 1 electron to form -1 anions. The anions are -1 because their 17 protons have a charge of +17, and their 18 electrons have a charge of -18, giving a sum of -1. The anion’s symbol is Cl-, again without the 1. As illustrated in Figure 11, oxygen atoms commonly form anions with a -2 charge, O2-, by gaining 2 electrons and therefore changing from eight protons and 8 electrons to 8 protons (+8) and ten electrons (-10).

Isotopes

Although all of the atoms of a specific element have the same number of protons (and the same number of electrons in uncharged atoms), they do not necessarily all have the same number of neutrons. For example, when the hydrogen atoms in a normal sample of hydrogen gas are analyzed, we find that of every 5000 atoms, 4999 have 1 proton and 1 electron, but 1 in 5000 of these atoms has 1 proton, 1 neutron, and 1 electron. This form of hydrogen is often called deuterium. Moreover, if you collected water from the cooling pond of a nuclear power plant, you would find that a very small fraction of its hydrogen atoms have 1 proton, 2 neutrons, and 1 electron (Figure 12). This last form of hydrogen, often called tritium, is unstable and therefore radioactive.

All of these atoms are hydrogen atoms because they have the chemical characteristics of hydrogen. For example, they all combine with oxygen atoms to form water. The chemical characteristics of an atom are determined by its number of protons (which is equal to the number of electrons if the atom is uncharged) and not by its number of neutrons. Because atoms are assigned to elements based on their chemical characteristics, an element can be defined as a substance whose atoms have the same number of protons. When an element has two or more species of atoms, each with the same number of protons but a different number of neutrons, the different species are called isotopes.

Atomic Number and Mass Number

The number of protons in an atom—which is also the number of electrons in an uncharged atom—is known as the element’s [atomic number]. The atomic number can be found above each of the elements’ symbols on the periodic table. Because it displays the atomic numbers, the periodic table can be used to determine the number of protons and electrons in an uncharged atom of any element. For example, the atomic number of phosphorus is 15, so we know there are 15 protons and 15 electrons in each uncharged atom of phosphorus.

The sum of the numbers of protons and neutrons in the nucleus of an atom is called the atom’s mass number. Isotopes have the same atomic number but different mass numbers. To distinguish one isotope from another, the symbol for the element is often followed by the mass number of the isotope. For example, the mass number of the most common isotope of hydrogen, with one proton and no neutrons, is 1, so its symbol is H-1. The other natural isotope of hydrogen, with one proton and one neutron, has a mass number of 2 and a symbol of H-2. Tritium, H-3, the radioactive form of hydrogen, has a mass number of 3. All of these isotopes of hydrogen have an atomic number of 1.

Nineteen of the elements found in nature have only one naturally occurring form. For example, all the aluminum atoms found in nature have 13 protons and 14 neutrons. Their mass number is 27.

Number of Protons + Number of Neutrons = Mass Number

13 + 14 = 27

The other naturally occurring elements are composed of more than one isotope. For example, in a sample of the element tin, Sn, all the atoms have 50 protons, but tin atoms can have 62, 64, 65, 66, 67, 68, 69, 70, 72, or 74 neutrons. Thus tin has 10 natural isotopes with mass numbers of 112, 114, 115, 116, 117, 118, 119, 120, 122, and 124 (Figure 13).

Common Elements

Most people look at a gold nugget and see a shiny metallic substance that can be melted down and made into jewelry. A chemist looks at a substance such as gold and visualizes the internal structure responsible for those external characteristics. Now that we have discussed some of the general features of atoms and elements, we can continue in our quest to visualize the particle nature of matter.

Gas, Liquid, and Solid Elements

We can picture gases as independent spherical particles moving in straight-line paths in a container that is mostly empty space. This image is most accurate for the noble gases (He, Ne, Ar, Kr, Xe, and Rn): each noble gas particle consists of a single atom. When we picture the helium gas in a helium-filled balloon, each of the particles in our image is a helium atom containing two protons and two neutrons in a tiny nucleus surrounded by a cloud of negative charge generated by two electrons (Figure 14).

The particles in hydrogen gas are quite different. Instead of the single atoms found in helium gas, the particles in hydrogen gas are pairs of hydrogen atoms. Each hydrogen atom has only one electron, and single, or "unpaired," electrons are less stable than electrons that are present as pairs. (Stability is a relative term that describes the resistance to change. A stable system is less likely to change than an unstable system.) To gain the greater stability conferred by pairing, the single electron of one hydrogen atom can pair up with a single electron of another hydrogen atom. The two electrons are then shared between the two hydrogen atoms and create a bond that holds the atoms together. Thus hydrogen gas is described as H2. We call this bond between atoms due to the sharing of two electrons a covalent bond. The pair of hydrogen atoms is a molecule, which is an uncharged collection of atoms held together with covalent bonds. Two hydrogen atoms combine to form one hydrogen molecule.

The negative charge-cloud created by the electrons in the covalent bond between hydrogen atoms surrounds both of the hydrogen nuclei (Figure 15).

Even though the shape depicted in Figure 15 is a better description of the H2 molecule’s electron cloud, there are two other common ways of illustrating the H2 molecule. The first image in Figure 16 shows a space-filling model. This type of model emphasizes individual atoms in the molecule more than the image in Figure 15 does but still provides a somewhat realistic idea of the electron-charge clouds that surround the atoms. The second image in Figure 16 is a ball-and-stick model, in which balls represent atoms and sticks represent covalent bonds. This model gives greater emphasis to the bond that holds the hydrogen atoms together.

Because hydrogen molecules are composed of two atoms, they are called diatomic. The elements nitrogen, oxygen, fluorine, chlorine, bromine, and iodine are also composed of diatomic molecules, so they are described as N2, O2, F2, Cl2, Br2, and I2. Like the hydrogen atoms in H2 molecules, the two atoms in each of these molecules are held together by a covalent bond that is due to the sharing of two electrons. Nitrogen, oxygen, fluorine, and chlorine are gases at room temperature and pressure, so a depiction of gaseous N2, O2, F2, and Cl2 would be very similar to the image of H2 in Figure 17. Combining space-filling molecular models with our gas model, Figure 17 depicts hydrogen gas as being very similar to helium gas, except each of the particles is a hydrogen molecule.

Solid iodine consists of a very ordered arrangement of I2 molecules. In order to give a clearer idea of this arrangement, the first image in Figure 19 shows each I2 molecule as a ball-and-stick model. The second image shows the close packing of these molecules in the iodine solid. Remember that the particles of any substance, including solid iodine, are in constant motion.

Metallic Elements

The metallic elements are used for a lot more than building bridges and making jewelry. Platinum is used in a car’s catalytic converter to help decrease air pollution. Titanium is mixed with other metals to construct orthopedic appliances, such as artificial hip joints. Zinc is used to make dry cell batteries. Some of the characteristics of metallic elements that give them such wide applications can be explained in terms of a model called the sea of electrons model. According to this model, each atom in a metallic solid has released one or more electrons, and these electrons move freely throughout the solid. When the atoms lose the electrons, they become cations. The cations form the structure we associate with solids, and the released electrons flow between them like water flows between islands in the ocean. This model, often called the sea of electrons model, can be used to explain some of the definitive characteristics of metals. For example, the freely moving electrons make metallic elements good conductors of electric currents.

Figure 20 shows a typical arrangement of atoms in a metallic solid and also shows how you might visualize one plane of this structure. Try to picture a cloud of negative charge, produced by mobile electrons, surrounding the cations in the solid.